The present invention is directed to ultrasonic transducers, in general, and more particularly to an improved ultrasonic transducer for measuring ultrasonically the quantity of liquid in a tank.
Ultrasonic liquid gauging systems, like a fuel gauging system for an aircraft, for example, generally include one or more ultrasonic transducers at each fuel tank of the aircraft, generally disposed at the bottom thereof, and one or more target reflectors disposed in the tank at predetermined distances from the ultrasonic transducer. In operation, an incipient ultrasonic burst signal is transmitted from the transducer, conducted through the liquid, reflected from the height of the liquid, i.e. the liquid/air interface, and returned to the transducer where it is received. A round trip time period from inception to reception of the ultrasonic burst signal is measured to determine the height of the liquid in the tank. In order to determine liquid height the velocity of sound of the liquid is needed. One technique for determining velocity of sound of the liquid is to utilize the time measurements for the ultrasonic burst reflections from the one or more target reflectors in the tank. Since the distance between a target reflector and the transducer is known the velocity of sound may be determined from said distance and the time measurement for the target reflector.
But this presumes that the velocity of sound of the liquid is substantially constant over a large liquid height profile around the target reflector. Unfortunately, this may not always be the case, especially if the liquid in the tank is thermally stratified. Accordingly, having the velocity of sound at one height of the liquid may not be sufficient across the over all height profile of the tank liquid, especially if accuracy of liquid quantity measurement is of paramount importance. Thus, it would be an important improvement to be capable of determining the velocity of sound cumulatively at the height of the liquid in the tank under thermally stratified conditions.
In addition, stratification may also occur due to a separation of different liquids in the tank. For example, reflections which may occur from the stratified liquid levels, may compromise the time measurements of the reflections from the target reflectors. Therefore, a liquid gauging system may also be improved by distinguishing between the different reflections in order to obtain accurate time measurements from the reflections of the target reflectors.
Also, current ultrasonic transducers like that illustrated in cross sectional view in FIG. 3A, for example, include a bottom layer of piezoresonator material which is of a different acoustic impedance than that of the liquid in the tank about the operational frequency passband of the ultrasonic burst or pulse transmitted and received therefrom. In some cases, this difference in acoustic impedance between the piezoresonator material and liquid may be greater than thirty to one, for example. Generally, one or more layers of material are disposed between the piezoresonator material and the tank liquid for matching the acoustic impedances of the piezoresonator material and the tank liquid to render an efficient energy transfer. Such impedance matching techniques are proposed in the following U.S. patents: Merewether, U.S. Pat. No. 5,343,443, issued Aug. 30, 1994; Breimesser et al., U.S. Pat. No. 4,672,591, issued Jun. 9, 1987; Rhyne, U.S. Pat. No. 5,706,564, issued Jan. 13, 1998; Mitchell et al., U.S. Pat. No. 4,396,663, issued Aug. 2, 1983; Kikuchi et al., U.S. Pat. No. 5,438,999, issued Aug. 8, 1995; and Seyed-Bolorforosh et al., U.S. Pat. No. 5,553,035, issued Sep. 3, 1996.
However, this acoustic impedance matching has not always been accurate due primarily to the available material for use in the additional impedance matching layer or layers. For example, Merewether (U.S. Pat. No. 5,343,443) proposes an anisotropic material for use as its primary acoustic impedance matching layer. Merewether""s anisotropic layer is a composite material of a polymer phenolic resin embedded with random oriented fibers of a particular material. Proposed materials for the fibers included quartz, graphite, carbon, Boron Nitride, and Silicon Carbide, for example. Focus appeared to be on having favorable coefficients of thermal expansion (CTEs) between layers, rather than overall efficiency in acoustic energy transfer through the transducer.
Layers made of composite material of a random oriented matrix are not homogeneous and include fibrous particles which tend to scatter, reflect back or dissipate acoustic energy and therefore, are very lossy. Also, such materials are very complex and difficult to manufacture; often resulting in inconsistent quality from one batch to another, and thus, not reliable. Also, the characteristics of such composite material are not consistent over a wide temperature range. In addition, the use of more than one layer for acoustic impedance matching tends to create further losses, especially in broadband applications. Accordingly, an improvement in efficiency of energy transfer can occur if the acoustic impedance matching is made more accurate than currently implemented.
The embodiment of the invention which will be described in a succeeding section ameliorates the aforementioned drawbacks, thus providing a more accurate and improved ultrasonic transducer for liquid measurement.
In accordance with one aspect of the present invention, an ultrasonic transducer comprises a layer of piezoresonator material having top and bottom surfaces and capable of transmitting from the top surface an ultrasonic pulse into a tank of liquid and receiving at the top surface reflections of said transmitted pulse from the liquid; and a matching layer of pure crystalline Boron Nitride disposed on the top surface of the piezoresonator layer, the ultrasonic pulse and reflections thereof conductible through the matching layer between the top surface of the piezoresonator layer and the tank liquid, the pure crystalline Boron Nitride layer operative to match the acoustic impedance of the piezoresonator material to the acoustic impedance of the tank liquid about the operational frequency passband of the ultrasonic pulse.
In accordance with another aspect of the present invention, an ultrasonic transducer assembly for measuring a quantity of liquid in a container comprises a housing having top and bottom surfaces, the top surface for interfacing with the liquid of the container; an ultrasonic transducer disposed in said housing and comprising: a layer of piezoresonator material having top and bottom surfaces and capable of transmitting from the top surface an ultrasonic pulse into the container of liquid and receiving at the top surface reflections of said transmitted pulse from the liquid, the top and bottom surfaces of the piezoresonator material covered with layers of conductive material; and a matching layer of pure crystalline Boron Nitride disposed on the top surface of the piezoresonator layer and configured as a window between the piezoresonator material and the liquid at the top surface of the assembly, the ultrasonic pulse and reflections thereof conductible through the matching layer between the top surface of the piezoresonator layer and the liquid, the matching layer operative to match the acoustic impedance of the piezoresonator material to the acoustic impedance of the liquid about the operational frequency passband of the ultrasonic pulse, a surface of the matching layer at the liquid interface being covered with at least one metal layer; and a lead wire for each surface of the piezoresonator layer connected at one end to the conductive material layer thereof, the lead wires connectable at the other ends to a transducer driver/receiver circuit.